In a preparation of organic light-emitting diode (OLED) devices, organic functional layer films are usually formed by an ink-jet printing method or the like, and in a preparation of liquid crystal display (LCD) devices, color filters are usually formed by the ink-jet printing method or the like. A common characteristic of various wet processes including ink-jet printing for forming a film lies in that: the excess solvent needs to be removed from the film by a subsequent drying process. The process for forming a dried film by removing the solvent will influence uniformity of the formed film. For photoelectric display devices, an uniformity of the dried film greatly influences the service life and the display effect of the display devices.
FIG. 1 schematically shows a top view of a display substrate including a dried film. As shown in FIG. 1, after the solvent ink, which contains organic functional material, is jetted onto a display area 41 of the display substrate 4, a process, in which the solvent is allowed to naturally volatilize at atmospheric pressure or is removed at a pressure lower than atmospheric pressure, is required. The solvent ink jetted onto the display area 41 is dispersed in a pixel area in a very small ink droplet form and has a large surface area, so that the natural volatilization rate of the solvent ink is high. The natural volatilization rate of a portion of the solvent ink located in an edge area 411 of the display area 41 is higher than that in a central area 412 of the display area 41.
FIG. 2 is a sectional view of the display substrate 4 in FIG. 1 taken along a cutting line 5. FIG. 2 shows pixel separation structures 42 and an organic film 43 after being resulting in non-uniform film formed after being dried (coffee-ring effect), in the pixel area of the edge area 411 of the display area. However, the film formed after being dried, in the central area 412 far from the edge area 411, has better uniformity. There are many reasons for the formation of the non-uniform film after naturally volatilizing and dried, for example, solvent characteristics composition ratio and vapor pressure, etc.), material of the pixel separation structure 42, equipment for implementation of the drying process, etc. In some cases, the non-uniform film will result in bad display of concentric circles extending from the center to the periphery, as shown in FIG. 1, after the display device is powered on.
In the existing production process, the solvent in the film formed by the wet process is usually removed in a vacuum furnace at a pressure lower than atmospheric pressure. An exhaust port of the current vacuum equipment is usually arranged at the corner of the furnace, easily resulting in non-uniform airflow distribution in an air exhausting process under reduced pressure, and the differences between the solvent volatilization rates in different areas of the film are changed with respect to the naturally volatilizing and drying condition, resulting in that non-uniformity of the dried film is also changed.
In a word, it is difficult to avoid non-uniform surface of the dried film, whether the solvent is removed from the film by the naturally volatilizing and drying method, or by using the conventional vacuum drying equipment. Therefore, how to reduce or avoid the non-uniform surface after drying the organic film formed by the wet process has become a difficult problem to be solved urgently.